This invention relates to a data transmission device which is operable in accordance with a protocol wherein a transmission packet size is adaptively variable in consideration of a bit error rate on a transmission line.
In order to define and implement communication between personal computers, a strong interest has been focused on Microcom Networking Protocol (often be abbreviated to MNP) which has been proposed by Microcom Corporation. Therefore, description will be mainly directed to the MNP, although this invention is never restricted to the MNP. The MNP is specified by detecting a transmission error by the use of a cyclic redundancy check technique and by automatically repeating a transmission data packet as long as no acknowledgement is received from a reception side. As a result, a communication system operable in accordance with the MNP may be collectively called an automatic repeat system.
Herein, it is to be noted that the MNP is classified into first through seventh classes in correspondence to modems to be used and additional functions assigned thereto. In the automatic repeat system of the first through third classes of the MNP, each input data signal is changed into a transmission data packet which carries an additional signal in addition to the input data signal. The additional signal may be, for example, a start flag, an end flag, and a check signal for checking an error and is therefore composed of a plurality of bits. Inasmuch as the additional signal is invariable in a bit number irrespective of a bit length, namely, a data size of the input data signal, a rate of the additional signal increases in a packet when the data signal has a short data size. This inevitably results in a reduction of transmission efficiency. In order to improve the transmission efficiency of a packet, it may be considered to make a data size long in each packet. However, a packet of such a long data size is susceptible to an error on a transmission line. As a result, the packet of a long data size must be repeatedly transmitted again and again. This might give rise to an increase of a repetition number of a packet and results in a reduction of total transmission efficiency. Thus antinomy takes place between a data size and transmission efficiency.
On the other hand, the fourth through seventh classes of the MNP are defined to adaptively vary a data frame length or a packet in consideration of a quality of a transmission line through which the transmission data packet is transmitted. The quality of a transmission line may be evaluated by a bit error rate, as well known in the art. In order to carry out communication in compliance with the fourth through seventh classes, it is necessary to adaptively decide an optimum packet size by monitoring a bit error rate in accordance with a preselected algorithm and to thereby obtain maximum transmission efficiency. Under the circumstances, a communication system according to the fourth through seventh classes of the MNP will be referred to as a variable packet system.
Herein, an optimum packet size must be decided by calculating a bit error rate from a repetition number of a packet by the use of the preselected algorithm. However, intricate calculations should be carried out to decide such an optimum packet size with real time because the preselected algorithm is usually specified by complicated nonlinear equations. Accordingly, an increase of hardware is inevitable for adaptively varying a packet size.